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Re: backbone routers' priority settings for ICMP & UDP

  • From: Scott Whyte
  • Date: Wed Feb 04 16:28:18 1998

On Wed, 4 Feb 1998, Dave Siegel wrote:

> Extremely weak metaphore, since a source quench indicates there weren't
> enough buffers available to send your packets.
> Now, if the freeway was full, and cars started dropping out of the space/time
> continuum, that'd be more like a source quench.  ;-)  The freeway would call
> your wife at home and say "sorry, but your husband didn't make it to work
> because the freeways were too full."  If wife runs correct a correct
> TCP implementation, she would know to initiate "slow start" and would 
> send out her husbands at a slower rate until she gets a feel for how
> bad the traffic is.

ROFL, nice extension.  But this is not true because, as you say below,
gateways don't source quench anyore.

> > One then
> > wonders how well Win95 implements source quench, if at all. 
> Which side of the implementation do you mean?  as a client, or as a gateway?
> I suppose it doesn't really matter.  Since source quenches are not supposed
> to be used on routers anymore, the expectation of receiving a source
> quench on a large network (like the Internet) is a bad one, so the TCP
> implementations have to implement congestion controls through other means
> anyhow.

As a client, of course, since end-to-end source quench is the only
alternative available.  And consider the near future scenario where a user
with a cable-modem connected via Ethernet to their nice new NC (with
cheapest bus design possible to contain costs) has requested a URL from a
<insert whomping fast server here> connected via OC-3 to the Internet.  It
seems likely a source quench will come in handy to provide flow control.

> TCP/IP Illus. Vol. I by W. Richard Stevens has a pretty good explanation
> of what source quenches are.

Don't have Mr. Stevens handy, but from RFC777 (1981!), when both types of
source quench were defined:

...A destination host may also send a source quench message if datagrams
arrive too fast to be processed.  The source quench message is a request
to the host to cut back the rate at which it is sending traffic to the
internet destination.

This is what I was getting at.  Flow control versus congestion control.
Scott Whyte 408.527.5713         |Any opinions expressed herein are 
Network Supported Accounts (NSA) |mine and not cisco's...
CCIE 3340                        |
                                 | "Eschew Obfuscation"